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Bromine water can test for an alkene, while carbonate reacts with a carboxylic acid to release carbon dioxide. Carbonyl compounds form a precipitate with 2,4-DNPH, but that alone does not distinguish aldehyde from ketone. Tollens’ reagent gives a silver mirror with suitable aldehydes, whereas ordinary ketones do not respond. Acidified dichromate can distinguish some oxidisable functional groups. Give reagent, conditions and observation; a named reagent without the expected evidence does not demonstrate identification.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Select chemical tests for functional groups
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does a positive 2,4-DNPH test indicate?
A carbonyl compound of the aldehyde/ketone type. Another test is needed to distinguish aldehydes and ketones.
After watching 2: Which region is useful for comparison with an authentic IR spectrum?
The fingerprint region. The detailed pattern can support identity.
What you'll be able to do
Bromine water can test for an alkene, while carbonate reacts with a carboxylic acid to release carbon dioxide. Carbonyl compounds form a precipitate with 2,4-DNPH, but that alone does not distinguish aldehyde from ketone. Tollens’ reagent gives a silver mirror with suitable aldehydes, whereas ordinary ketones do not respond. Acidified dichromate can distinguish some oxidisable functional groups. Give reagent, conditions and observation; a named reagent without the expected evidence does not demonstrate identification.
Infrared absorption occurs when a vibration changes a molecule’s dipole moment. Use the provided wavenumber data to identify features such as C=O and O–H. The broad O–H absorption of a carboxylic acid differs from an alcohol’s O–H region. The fingerprint region helps compare a spectrum with a known compound but is difficult to interpret bond by bond. An absent O–H absorption can rule out some candidates; a carbonyl peak alone cannot uniquely distinguish every carbonyl-containing functional group.
A molecular-ion peak gives the relative molecular mass for a singly charged molecular ion. Fragment peaks indicate stable ions produced by bond cleavage; neutral fragments are not detected directly. A mass difference can suggest loss of a group, but several structures can share a fragment. Isotope patterns can indicate chlorine or bromine. Combine molecular formula, chemical tests and infrared results to narrow candidates, and explain which observation excludes each alternative. One matching peak rarely establishes a unique structure.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A singly charged molecular-ion peak is at m/z 58. Can this alone prove propanone?
No; further functional-group and structural evidence is needed.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.